1. Field of the Invention
The present invention relates to an apparatus and method of controlling a position of a rotor in a high pressure feeder.
More particularly, the present invention relates to an apparatus and method of controlling a position of a rotor in a truncated conical high pressure feeder to prevent wear of the rotor and reduce the exchange frequency of the rotor.
2. Description of the Related Art
A continuous digester is widely employed as an apparatus for producing pulp from a lignocellulose material by an alkaline pulping process.
A feed comprising lignocellulose material such as wood chips and an alkaline cooking liquor is fed into the pulping digester through a high pressure feeder, usually a truncated conical rotary feeder.
The high pressure feeder serves to feed the lignocellulose material (wood chips) together with the alkaline cooking liquor to the continuous digester, and to seal the continuous digester from the ambient atmosphere so as to maintain the pressure in the continuous digester at a high level. Accordingly, to maintain a high pressure in the continuous digester, a tight seal between the casing of the high pressure feeder and the rotor in the feeder is very important.
Usually, the high pressure feeder comprises a truncated conical rotor, a casing receiving the rotor therein, and an electric motor for driving the rotor. This rotor-driving motor is arranged at a small diameter end side of the rotor and close to a rotating shaft of the rotor.
During the movements of the rotor shaft in the high pressure feeder, the rotor moves in the direction of the small diameter end thereof by pushing the rotor shaft inward, and in the direction of the large diameter end thereof by pulling the rotor shaft outward.
During the feeding operation, the outside surface of the rotor of the high pressure feeder is worn away. The main reasons for the wear reside in the rotation of the rotor producing friction between the outside surface of the rotor and the inside surface of the casing, and the unavoidable sticking of the lignocellulose material (wood chips) containing foreign substances such as sand or metallic pieces into the space between the rotor and the casing.
Due to the wear of the outside surface of the rotor by friction produced between the rotor and the casing, and sticking of foreign substances into the space between the rotor and the casing, the clearance between the rotor and the casing is gradually enlarged.
When the clearance is enlarged, it becomes impossible to completely seal the high pressure feeder and thus the continuous pulping digester cannot be smoothly operated. Accordingly, to adjust the clearance to an appropriate level, the rotor is moved by pushing the rotor shaft inward.
As mentioned above, to stably operate the continuous pulping digester, it is indispensable to maintain an appropriate clearance between the rotor and the casing by pushing the rotor shaft inward. In a conventional operation, the pushing operation of the rotary shaft is carried out manually in accordance with the experience of the operator. However, since the clearance is very small, it is difficult to carry out the pushing operation of the rotor shaft and accurately adjust the clearance. Therefore, the pushing operation of the rotary shaft is often inappropriately carried out and the inside surface of the casing and the outside surface of the rotor are undesirably worn by excessive pushing of the rotor shaft or by allowing the sticking of foreign substances into the space between the casing and the rotor.
Japanese Unexamined Patent Publication (JP-B-) No. 1-239,184 discloses an apparatus and a method of automatically controlling an electric motor for moving a truncated conical rotor in response to a change in the load torque of the motor, thereby reducing the wear rate of the rotor.
The inventors of the present invention studied the wear of the rotor, the types of foreign substances in the lignocellulose material (wood chips), and the change in the amount of foreign substances with time.
Namely, the inventors extracted the foreign substances from wood chips by using a foreign substance-separator having a sand trap and a metal trap, and measured the weight of the collected foreign substance, at time intervals of 4 hours over a period of 30 days. From this study it was found that the amount of foreign substances contained in the wood chips varied in a range of from 1.2 kg to 54.4 kg per 4 hours.
Also, the inventors found that when a mixture of various cargoes of wood chips collected from various regions was supplied to a continuous pulping digester, and the mixing proportions of the various cargoes of wood chips were constant, the content of the foreign substances in the mixed wood chips was substantially constant.
Further, the inventors found that when the content of the foreign substances in the mixed wood chips was constant, the wearing rate of the rotor in the high pressure feeder was substantially constant, even if the operational conditions of .the high pressure feeder, for example, the number of revolutions of the rotor and the feeding rate of the wood chips (absolute dry weight/day) varied.
Furthermore, the inventors of the present invention found that in the operation of the continuous pulping digester, a change in the number of revolutions of the rotor frequently causes a change in the feeding rate of the wood chips, which corresponds to the production rate of the pulp by the digester, and a change in the flow rate of an alkaline cooking liquor supplied under pressure to the high pressure feeder causes the load torque of the rotor-driving electric motor to vary, and therefore, if the rotor shaft-pushing operation is controlled in response to a change in the load torque of the rotor-driving electric motor, the rotor shaft was sometimes automatically pushed or pulled even when it was unnecessary. Accordingly, the above-mentioned controlling method and apparatus, in which the rotor shaft-pushing operation is controlled in response to a change in the load torque of the rotor-driving motor, were not expected to reduce the wear rate of the rotor and thus it is difficult to affirm that this technique is useful from a practical point of view.
Under these circumstances, there is a strong demand to provide a new control apparatus and method for the rotor in the truncated conical high pressure feeder, that appropriately reduces the wear rate of the rotor.